This invention relates to discharge resistors, and more particularly to such resistors for use in dynamo-electric machines such as brushless synchronous motors.
In brushless synchronous motors, rotating field coils of the motors are connected through rectifying devices to rotating armature coils of exciters. The rotating field coils of the motors, the rectifying devices, and the rotating armature coils of the exciters rotate in unison, and the field current in such field coils is controlled by controlling the current in the stationary field coils of the exciters. However, during start up of the motors, in which the motors run in an asynchronous manner, i.e., out of synchronization, relatively high a-c voltage appears in the field coils of the motors, as is well-known in the art. Such voltage occasionally may rise to over 10,000 volts, although this will depend on the size of the motor. Such relatively high voltage also tends to appear upon a sudden change in the inner phase angle of synchronous motors due to the abrupt change in the mechanical load thereof. Accordingly, it is necessary to protect the field coils themselves, and also the rectifying devices, from suffering adverse effects such as insulation breakage due to such high voltage.
To provide the necessary protection, discharge resistors, i.e., starting resistors, are usually mounted on the shaft of the motors in such a manner as to shunt the field coils. There are currently two methods for connecting the resistor in parallel with the field coil. According to one method, the discharge resistor is connected to shunt the field coil only during start up of the motor. This method can be employed when it is expected that any damage to the motor due to abnormal voltage in steady state operation (which may appear upon a sudden change in the inner phase angle of the synchronous motor) will be negligible.
In the other method, the discharge resistor may be connected to shunt the field coil of the motor at any time.
Desirable basic features for discharge resistors include the following:
(a) the resistivity required for pertinent protection can be easily obtained;
(b) they have substantially no inductive impedance;
(c) they have predetermined heat dissipation ability;
(d) they are durable against ambient conditions, i.e., they are substantially free from corrosion, oxidization, insulation wear, and the like; and
(e) they are of simple construction and can withstand the centrifugal forces present during operation.
Typical prior art discharge resistors are comprised of disk-like metal casings filled with insulating material in which resistor wires, such as nichrome wires, are buried. As can be easily understood, such resistor wires are very difficult to obtain in the desired length, and, in turn, the desired resistivity. The disk-like metal casings are adapted to be mounted on the motor shaft to keep the insulating material, and the resistor wires buried therein, in a predetermined position when the shaft rotates. In this prior art system, the surface area of the resistor wires through which heat generated by the resistor dissipates is relatively small, so that the temperature of the resistor rises relatively high, e.g., from 400.degree. C. to 1000.degree. C. It has thus been necessary to use insulating material capable of withstanding relatively high temperatures. This, in turn, means that the resistor wire must be surrounded by a considerable amount of insulating material, since material having very good insulation characteristics, such as organic insulating material, generally has relatively poor durability against temperature rise and cannot, therefore, be used. Surrounding the resistor wire by large amounts of insulating material also causes the rate of heat transfer to be low, thereby resulting in a greater temperature rise in the resistor wire. There are disadvantages such that, for example, the volume and weight of the resistor is relatively large, thereby causing the motor to be larger. In case of high speed motors, particularly, it may be necessary to provide a third journal rotatably supporting the motor shaft for safety purposes. Dielectric strength of inorganic insulating material is also apt to decrease due to absorption of moisture. From these disadvantages, freedom in designing the resistors is relatively small, so that it becomes difficult to obtain non-inductive resistance.
Regarding the resistors used to shunt the field coils of the motors only during start up, temperature rise in the resistor depends mainly on the heat capacity, Q/(W C), of the resistor wires, where Q is generated heat, W is weight of the resistor wire, and C is specific heat of the resistor wire. That is, it is necessary to increase the weight W of the resistor wire to decrease the temperature rise in this type of application.
To solve this problem, resistors have been developed which include a strip-like resistor member which is wound edgewise and folded back at each turn to cancel inductivity, with clearances between adjacent turns. Such a resistor member is coaxially supported by frame member supports. This structure, however, is difficult to manufacture, particularly in connection with the edgewise winding of the resistor strip.